[Bug 1783597] Re: The disk-space-needed estimate doesn't account for the snaps

[Łukasz Zemczak]

Verified ubuntu-release-upgrader 19.04.16.7 as per the test-case.

I set up a multipass bionic system, switched /etc/update-manager
/release-upgrades to Prompt=normal, installed ubuntu-desktop, installed
the core18 snap (from stable) and gnome-characters snap (from
stable/ubuntu-18.04). I then proceeded with running `do-release-upgrade
--frontend DistUpgradeView --proposed`. The upgrade succeeded, I have
confirmed that 19.04.16.7 was used [1], checked that all snaps have been
refreshed correctly. Also, checked that '/var' is listed in the snap
estimates [2].

Will still check if update-manager is fine before switching the tag to
verification-done.

[1] 2019-10-09 13:28:16,936 INFO release-upgrader version '19.04.16.7' started
[2] 2019-10-09 13:29:05,882 DEBUG dir '/var' needs '9207808' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f837003a278>' (9367467576.000000)

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Title:
  The disk-space-needed estimate doesn't account for the snaps

Status in ubuntu-release-upgrader package in Ubuntu:
  Fix Released
Status in ubuntu-release-upgrader source package in Disco:
  Fix Committed
Status in ubuntu-release-upgrader source package in Eoan:
  Fix Released

Bug description:
  [Impact]

  Since there have been reports that users might fail to boot to a
  working gdm in the case of a full disk, we need to make sure users
  upgrading from and older release to disco/eoan have enough space for
  the upgrade to succeed. Currently we are installing snaps to replace
  certain debs on the system (+ refreshing existing snaps to newer
  channels/branches). The additional size required for the snap
  replacements was not taken into consideration when performing the
  upgrade, which might lead to edge-cases of users not having enough
  space for the upgrade to succeed.

  [Test Case]

  * Prepare a bionic machine
  * Make sure ubuntu-desktop and snapd are installed on it
  * Install core18 from stable and, for instance, gnome-characters from the stable/ubuntu-18.04 branch
  * Do snap list and snap refresh other gnome-related snaps to stable/ubuntu-18.04
  * Perform an upgrade to disco (with the --proposed option, to use disco-proposed)
  * Reboot
  * Make sure the update succeeded and all the installed gnome snaps now track stable/ubuntu-19.04 instead
  * Check /var/log/dist-upgrade/main.log and see if '/var' is listed in the calculated required space print-out

  Additional test case to make sure update-manager did not regress due
  to the changes:

  * On the newly upgraded system (or a new disco instance with ubuntu-release-upgrader upgraded to the version in disco-proposed), downgrade any of the installed packages
  * On the terminal, run `update-manager` and wait for the window to appear
  * Once the list of available upgrades appears, press the 'install now' button
  * Upgrades should install and no python traceback should be printed on the terminal

  [Regression Potential]

  As with any change touching the upgrade code-path, this change also carries a risk and should be well tested before going forward. That being said, the fix has been specifically basically limited to DistUpgradeQuirks to make regressions less likely. Additionally, unit tests have been added for every modified code-path.
  If any regressions should be found, the most likely place for those would be in size estimation, causing users to get notified of lack of space when they shouldn't. Another possible location would be all the things regarding replacing the debs with snaps - so either failures in snap replacement installation or refreshing.

  [Original Description]

  Looking at the code change recently made to install snaps on upgrade,
  it seems like there was no change made to the disk-space-estimate
  code. We had some reports recently that bionic might fail to boot to a
  working gdm on full disk so that could potentially be an important
  issue for upgraders

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